The invention relates to an implant for fixing an osteotomy, in particular for treating an axial deviation of a metatarsal bone, e.g. hallux valgus, and consisting of a clasp with two arms which are connected to each other at one of their ends.
Osteotomies for treating hallux valgus have already been known for decades and have the object of anatomically reconstructing the axis of the 1st metatarsal. It is necessary in this context, after repositioning the osteotomy, to fix the two bone fragments in their repositioned state in order to prevent mobility between the fragments and to permit primary bone healing without callus forming as a result of movement. There are various techniques for this fixing.
Thus, it is already known to connect the two bone fragments to each other by means of a screw which is introduced into a pre-drilled slide hole in the proximal fragment and is screwed into a threaded hole in the distal fragment. It is also already known to use self-tapping screws or hollow screws which permit a temporary fixation of the osteotomy by means of Kirschner wires belonging to the system. The use of these screws causes substantial inter-fragment compression, but this has not proven to be of any advantage since a pressure of more than 200 KTa (30 psi) leads to necrosis of the cortical bone.
It has already been proposed to fix an osteotomy by means of a wire which is pushed through pre-drilled holes in the osteotomy, whereupon the wire ends are twisted and the twisted area is embedded in a separate hole in the bone. This type of fixing is involved and time-consuming.
Fixing the osteotomy with a drill wire protruding from the skin causes temporary immobilization of the basal joint of the big toe, which is a disadvantage.
Bone clamps are also known for fixing the osteotomy, but their use involves the risk of splintering of the bone.
Also used for fixing the osteotomy are plates which have to be secured to the cortical bone via a number of screws, with the result that more bone mass is lost, and considerable surgical outlay is necessary for this.
An osteosynthesis clasp is known (FR-A-2722545) which consists of two parallel arms which are connected to each other in a U shape via a single arch without reverse point. One bone fragment is gripped using the arch of the clasp, and the free ends of the arms are inserted into the other fragment, where they are intended to be maintained by virtue of their undulation, but this is not sufficiently reliable.
Starting from the last-mentioned prior art, the object of the invention is to make available a clasp for fixing an osteotomy in accordance with the preamble of claim 1, which clasp is particularly suitable for treating an axial deviation of a metatarsal bone and ensures a sufficiently stable fixing of the osteotomy, can be fitted easily and without irritations, can be anchored in the bone and can also be removed again in a manner which is well tolerated by the patient.
The solution according to the invention lies in the features of claim 1. According to the latter, the two arms, in the area of their ends connected to each other, delimit an eyelet for the passage of a screw which can be anchored in one of the two bone fragments. The other, free ends of the arms can spread open in the medullary cavity of the other bone fragment. This spreading of the arms in the medullary cavity of the bone is achieved by the fact that the distance between the two arms made of a flexible elastic material, for example steel wire, increases, at least in the unloaded, relaxed state, from the connection point in the direction towards their free ends. Upon introduction of the arms into the medullary cavity, they are pressed together and then, as a result of their flexible elastic properties, brace themselves on the wall of the medullary cavity when the pressure exerted on the arms ceases.
After osteotomy has been performed, the clasp is introduced proximally into the medullary cavity via the free ends of its two arms and braces itself there in the medullary cavity of the one bone fragment, after which the end of the clasp provided with the opening is anchored on the other bone fragment by means of the screw. Through this intramedullary anchoring of the clasp by means of the spread-out ends of the arms, on the one hand, and the screwing-tight of the clasp, on the other hand, the osteotomy acquires a sufficiently stable fixing and permits immediate loading and early mobilization with a hallux mobility splint. Costly wound care and restraint dressings which limit mobility are no longer necessary.
From the fourth week after the operation, the clasp and the screw can be removed using a stab incision, which procedure is well tolerated by the patient.
The screw eyelet is expediently designed as an eyelet with a contour essentially in the shape of an arc of a circle, and dimensioned so that the screw can be passed through this opening, but so that an undesired lateral movement of the screw inside the opening is nevertheless prevented, and the screw head exerts contact pressure on the bone fragment.
It is also of advantage if those ends of the two arms connected to each other and delimiting the opening are bent outwards from the arm plane set by the remaining part of the arms and lie in a plane approximately parallel to this arm plane. The distance between these two planes takes account of the lateral offset of the bone fragments.
The intramedullary anchoring can be improved by the fact that according to the invention the two arms have an undulation, preferably running in the arm plane.
However, the clasp can also be made of a memory metal, preferably a nickel/titanium alloy. Such memory metals are known and are also used for implants. These memory metals have a property which is such that in the event of mechanical deformation, they retain their shape effected by this mechanical deformation below certain temperatures, but return to their original shape when the temperature rises. Thus, if the clasp is made of a memory metal, the two ends of the arms can be brought towards each other after cooling the clasp, for example by spraying it with a cooling gas, and they initially maintain this shape. After the arms have been introduced into the medullary cavity, the temperature of the clasp rises to body temperature, whereupon the clasp made of memory metal again assumes its original shape, with the ends of the arms spread apart, and is thus braced on the wall of the medullary cavity.